1. Field of the Invention
The present invention relates to an echo preventing circuit and a digital signal processing circuit.
2. Description of the Related Art
Recently, for example, each of some communicating apparatuses such as a mobile phone and a hands-free phone that can each be connected to an earphone microphone, incorporates an echo preventing circuit to prevent echoes generated due to sound coupling that travels along from a speaker to a microphone, electric reflection in a circuit, etc. For example, Japanese Patent Application Laid-Open Publication No. 3293029 discloses a circuit that prevents echoes by canceling an input signal using a signal having a phase that is an inverse of that of the input signal and having the amplitude level equal to that of the input signal. The configuration disclosed in the above '029 publication needs to set the circuit constants of the circuit elements with high precision to cancel echoes with high precision. However, such a configuration is not easy and this circuit can not cancel echoes with high precision.
Therefore, a method of canceling echoes with high precision using digital processing is considered. FIG. 51 depicts an example of an echo preventing circuit that uses a DSP 400. As depicted, an analog signal indicating sound transmitted from a counterpart using a mobile phone, etc., is inputted into an AD converter 401. The signal converted into a digital signal by the AD converter 401 is applied with convoluting processes respectively by FIR filters 402 and 403 in the DSP 400 respectively based on filter coefficients thereof, and is outputted. A signal outputted from the FIR filter 402 is inputted into a DA converter 404. The signal converted into an analog signal by the DA converter 404 is outputted to an earphone microphone through an input/output terminal 405 and is inputted into one of two terminals of a differential amplifying circuit 406. The signal outputted from the FIR filter 403 is inputted into a DA converter 407. The signal outputted from the DA converter 407 is inputted into the other terminal of the differential amplifying circuit 406.
The signal outputted from the differential amplifying circuit 406 is converted into a digital signal by an AD converter 408 and is inputted into the DSP 400. After being outputted from the DSP 400, this digital signal is converted into an analog signal by a DA converter 409 and is outputted as an output signal of the echo preventing circuit.
The DSP 400 acquires impulse responses of a section from the DA converter 404 to the AD converter 408 based on the output of the AD converter 408 obtained when the DSP 400 outputs an impulse to the DA converter 404. The DSP 400 acquires impulse responses of a section from the DA converter 407 to the AD converter 408 based on the output of the AD converter 408 obtained when the DSP 400 outputs an impulse to the DA converter 407. By setting properly the filter coefficients of the FIR filters 402 and 403 based on these impulse responses, echoes can be canceled with high precision.
Although the configuration shown in FIG. 51 can cancel echoes with high precision, the configuration needs two AD converters and three DA converters and, therefore, causes increase of the cost and the power consumption thereof.